Hot melt adhesives are becoming increasingly widely used to secure substrates together in a great variety of applications. These materials are essentially solvent-free adhesives which are applied in a molten state and form a bond upon cooling to a solid state. By reason of their quick-setting characteristics, their adhesive "tack", and their gap-filling properties, they are highly useful for a myriad of industrial adhesive applications.
Because of their rather short "open time" after application, while they are sufficiently fluid to be receptive for adhering a substrate, hot melt adhesives must be applied rapidly and the closure effected quickly. They are very viscous, even as melted, and it has therefore been the usual practice to apply them by extrusion as a line or bead, directly onto the substrate to be bonded.
In many sealing, laminating, or "gap-filling" operations it is desirable to apply adhesive to the substrate at a high rate of speed, in an adhesive pattern in which the adhesive lines on the substrate extend transversely or at right angles to one another. As an example, one such situation arises in the manufacture of paper bags, in the closure of the bag bottom flap structure. As shown in Hayward et al U.S. Pat. No. 2,864,549, and in Bedore U.S. Pat. No. 3,645,815, an adhesive is applied in a "U-shaped" pattern to the bag end flap structure forming a block-U or block-C shape, following which the flaps are folded over and sealed together. After such folding and sealing, this adhesive pattern provides a continuous line or bead of glue along the edges of the flap and thereby forms a "sift-proof seal". Such complete sealing is desirable to eliminate any minute channels or openings through which granular material stored in the bag could filter out.
At the present time, in making such seals, an ordinary cold-setting adhesive is applied in the desired U-shaped pattern while the bag is traveling on a conveyor at a rate of 150-300 feet per minute, using a so-called "paster wheel" to apply the cold glue. A raised area on the wheel surface picks up the glue from a reservoir and carries it to the moving bag where the rolling movement of the wheel relative to the bag transfers the glue in the U-shaped pattern.